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Do leaky tone caps effect tone?

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  • Do leaky tone caps effect tone?

    I have had two amps in recently with one leaky tone stack cap, causing dc on some pots, and scratchiness. Replacing them resolved the issue, but I am wondering if and how this may effect tone? One of the amps went home last week, and I got a text saying that it sounded fantastic, so I wonder if replacing the cap had something to do with that. Seems like having 3 - 4 volts sitting on the preamp signal can't help.
    It's weird, because it WAS working fine.....

  • #2
    Of course leaky caps effect tone. A leaky cap is no longer a cap. It's a resistor. You can't expect the circuit to work correctly.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      Consider that following any voltage division in the tone stack whatever DC remains is on the following stages grid. Since preamp tubes typically bias under 2V it doesn't take much to throw it off.
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "If you're not interested in opinions and the experience of others, why even start a thread?
      You can't just expect consent." Helmholtz

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      • #4
        In any case, it's definitely not working as the designer intended!

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        • #5
          Originally posted by Chuck H View Post
          Consider that following any voltage division in the tone stack whatever DC remains is on the following stages grid. Since preamp tubes typically bias under 2V it doesn't take much to throw it off.
          True 'nuff & let's also consider stages where there is no tone stack - output tube grid drives, input to splitter/inverter, reverb mix, etc. NONE of them like to be misbiased. So - stop the leakers & enjoy good tone, the way it was meant to be. Also, leaky caps have a tendency to not leak at a steady rate IOW they create noises we don't want to hear. Often, but not always.

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          This isn't the future I signed up for.

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          • #6
            Yeah, if you wanna look at in terms of percentages, takeca typical triode stage that wants to be biased at -1to2 volts... Add a leaky coupling cap at +1to2 volts, thatsa pretty hefty misbias. Not only affects tone, but can blow up some stuff, probably a minute or so right after it sounded better than ever! At least, that's the way Jack Darr explained it in 1972.

            Kinda the effect of biasing your power tubes at 150-200% dissipation...

            Justin
            "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
            "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
            "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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            • #7
              Originally posted by Justin Thomas View Post
              Yeah, if you wanna look at in terms of percentages, takeca typical triode stage that wants to be biased at -1to2 volts... Add a leaky coupling cap at +1to2 volts, thatsa pretty hefty misbias. Not only affects tone, but can blow up some stuff, probably a minute or so right after it sounded better than ever! At least, that's the way Jack Darr explained it in 1972.

              Kinda the effect of biasing your power tubes at 150-200% dissipation...

              Justin
              With a large value resistor standing in the way, in the plate circuit, probably won't wreck most preamp tubes. Nonetheless, they're best operated where they expect to be.
              This isn't the future I signed up for.

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              • #8
                I suppose I already figured it would effect tone somehow, but I was really wondering if it is something that could be identified, like the scratchy pot clue. Or in other words, does a leaky preamp cap have an identifiable characteristic sound?

                Or should I just shut up and go lick doorknobs?
                It's weird, because it WAS working fine.....

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                • #9
                  It's a bad time to be licking doorknobs!
                  "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                  • #10
                    Originally posted by Randall View Post
                    does a leaky preamp cap have an identifiable characteristic sound?

                    Or should I just shut up and go lick doorknobs?
                    Geeze, I wouldn't suggest that, even without filthy dog germs present everywhere!

                    Bad caps can emulate all the noises you'd think a bad tube can make - squeeks, pops, outer-space noises - just about everything but hum. And the effects on tone can vary quite a bit, from sounding thin & weedy to dull & lackluster. Also splodgy, a sort of annoying indistinctness.

                    Now let the dog lick doorknobs. Or kiss Lucy...

                    Public Service Announcement! It's gonna take some funny stuff to get us out of this plague alive. So - when the pols & pundits are running their mouths, think of this instead:

                    This isn't the future I signed up for.

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